Method for reducing formation permeability with gelled polymer solution having delayed gel time

ABSTRACT

The permeability of a subterranean formation is reduced by mixing a polymer, water, and a water soluble compound of a multivalent metal capable of furnishing multivalent metal ions to form a first mixture, thereafter mixing a complexing agent with the first mixture to form a second mixture, thereafter mixing a reducing agent with the second mixture to form a third mixture, and passing the third mixture having a delayed or extended gel time into the subterranean formation.

/ United States Patent 11 1 Hessert et a1.

[ 1 Dec. 16, 1975 METHOD FOR REDUCING FORMATION PERMEABILITY WITH GELLEDPOLYMER SOLUTION HAVING DELAYED GEL TIME [75] Inventors: James E.Hessert; Chester C.

Johnston, Jr., both of Bartlesville, Okla.

[73] Assignee: Phillips Petroleum Company,

Bartlesville, Okla.

221 Filed: Feb. 12,1974

211 Appl. No.: 441,848

Related U.S. Application Data [63] Continuation-impart of Ser. No.318,847, Dec. 27,

1972, abandoned.

[52] U.S. Cl. 166/294; 166/295; 166/275 [51] Int. C1. E21B 33/138; E21B43/22 [58] Field of Search 166/294, 295, 270, 274, 166/275, 246;252/316; 175/65, 72

[56] References Cited UNITED STATES PATENTS 3,208,524 9/1965 Horner eta1. 166/294 3,241,612 3/1966 Hiller 166/294 X 3,306,870 2/1967 Eilers etal 166/295 X 3,502,149 3/1970 Pence, Jr. 166/295 3,611,733 10/1971Eilers et a1 166/294 X 3,658,129 4/1972 Lanning et 211......" 166/2703,687,200 8/1972 Routson 166/295 X 3,749,172 7/1973 Hessert 166/270 X3,749,174 7/1973 Friedman et a1 166/294 Primary Examiner-Stephen J.Novosad 5 7] ABSTRACT 14 Claims, No Drawings METHOD FOR REDUCINGFORMATION PERMEABILITY WITH GELLED POLYMER SOLUTION HAVING DELAYED GELTIME This application is a continuation-in-part of our copendingapplication Ser. No. 318,847, filed Dec. 27, 1972 now abandoned.

In the art of producing hydrocarbons from a subterranean hydrocarboncontaining formation, it is often desirable to reduce the permeabilityof preselected portions of the formation. One example of where it isdesirable to reduce the permeability of the formation would be in thatportion of the formation adjacent a well bore which forms a pathway forwater to pass from the formation into the well bore. Another example isdecreasing the permeability of formations prior to or during secondaryrecovery operations, such as fluid drive processes.

One of the problems often encountered in utilizing gelable solutions ofpolymeric materials to reduce the permeability of subterraneanformations is delaying or extending the gel time sufficiently to providesufficient time for passing the gelable material downwardly through awell bore and through the formation to a desired location in theformation.

This invention therefore resides in reducing the permeability of asubterranean formation by mixing a polymer, water, and a water solublecompound of a multivalent metal capable of furnishing multivalent metalions to form a first mixture, thereafter mixing a complexing agent withthe first mixture to form a second mixture, thereafter mixing a reducingagent with the second mixture to form a third mixture, and passing thethird mixture having a delayed or extended gel time into thesubterranean formation.

In the method of this invention, a polymer, a water soluble compound ofa multivalent metal capable of furnishing multivalent metal ions, andwater are mixed together to form a first mixture.

Polymers that have been foundto be particularly useful in this processare polyacrylamides, polysaccharides, cellulosic polymers, and mixturesthereof. Examples of these polymers are Polyacrylamide Dow Pusher 700;Dow Pusher 1000 Polyfloc 1160 Betz HiVis Polysaccharide Kelzan MFManufactured by Dow Chemical Company, Midland, Michigan Manufactured byBetz Laboratories, Inc., Trevose, Pennsylvania 19047. See US. Patents3,208,524 and 3,383,307 Manufactured by Kelco, Inc., San Diego,California U.S. Patent 3,000,790. Cellulosic polymers (water soluble)Alkyl and hydroalkyl cellulose derivative Carboxymethyl cellulose (CMC)Carboxyethyl cellulose Carboxymethyl hydroxyethyl cellulose (CMHEC)propyl cellulose; alkyl cellulose such as methyl cellulose, ethylcellulose, and propyl cellulose; alkylcarboxyalkyl cellulose such asethylcarboxymethyl cellulose; alkylalkyl celluloses such as methylethylcellulose; and hydroxyalkylalkyl celluloses such as hydroxypropylmethylcellulose; and the like. Many of said cellulose ethers are availablecommerically in various grades. The carboxy-substituted cellulose ethersare avaiable as the alkali metal salt, usually the sodium salt. However,the metal is seldom referred to and they are commonly referred to asCMC, CMHEC, etc. For example, water-soluble CMC is available in variousdegress of carboxylate substitution ranging from about 0.3 up to themaximum degree of substitution of 3.0. In general, CMC having a degreeof substitution in the range of 0.65 to 0.95 is preferred. FrequentlyCMC having a degree of substitution in the range of 0.85 to 0.95 is amore preferred cellulose ether. CMC having'a degree of substitution lessthan the above-preferred range is usually less uniform in properties andthus less desirable for use in the practice of the invention. CMC havinga degree of substitution greater than the abovepreferred ranges usuallyhas a lower viscosity and more is required in the practice of theinvention. Said degree of substitution of CMC is commonly designated inpractice as CMC7, CMC-9, CMC12, etc., where the 7, 9, and 12 refer to adegree of substitution of 0.7, 0.9, and 1.2, respectively.

In the above-described mixed ethers, it is preferred that the portionthereof which contains the carboxylate groups be substantial instead ofa mere trace. For example, in CMHEC it is preferred that thecarboxymethyl degree of substitution be at least 0.4. The de- 4 gree ofhydroxyethyl substitution is less important and can vary widely, e.g.,from about 0.1 or lower to about 4 or higher.

Included among the polysaccharides which can be used in the practice ofthe invention are the biopolysaccharides produced by the action ofbacteria of the genus Xanthomonas on carbohydrates. These materials arethus biochemically synthesized polysaccharides and can be referred to asbi'opolysaccharides to distinguish them from naturally occurringpolysaccharides. Representative species of said Xanthomonas bacteriainclude Xanthomonas begoniae, Xanthomonas campestris, Xanthomonascarotae, Xanthomonas corylina, Xanthomonas gummisudans, Xanthomonashederae, Xanthomonas incanae, Xanthomonas lespedezae, Xanthomonasmalvacearum, Xanthomonas holcicola, Xanthomonas papavericola,Xanthomonas phaseoli, Xanthomonas pisi,

Xanthomonas translucens, Xanthomonas vasculorum and Xanthomonasvesicatoria. It has been shown in the prior art that the production ofsuch biopolysaccharides is a characteristic trait of members of theXanthomonas genus. Certain species produce the polymers with particularefiicency and are thus preferred. These preferred species includeXanthomonas begoniae, Xanthomonas campestris, Xanthomonas incanae, andXanthomonas pisi.

A wide variety of carbohydrates can be fermented with bacteria of thegenus Xanthomonas to produce said biopolysaccharides. Suitablecarbohydrates include sucrose, glucose, maltose, fructose, lactose,galactose, soluble starch, corn strach, potato starch, and the like. Theprior art has also shown that the carbohydrates need not be in a highlyrefined state and that crude materials from natural sources can beutilized. Examples of suitable such natural source materials in- .3clude crude molasses, raw sugar, raw potato starch, sugar beet juice,and the like. Since they are much less expensive than the correspondingrefined carbohydrates, such natural source materials are frequentlypreferred for use as the substrate in preparing said biopolysaccharides.

Fermentation of the carbohydrate to produce said biopolysaccharides canbe carried'out in an aqueous medium containing from about 1 to 5 percentof the carbohydrate, from about 0.1 to 0.5 weight percent of dipotassiumacid phosphate, and from about 0.1 to weight percent of a suitablenutrient containing suitable trace elements and organic nitrogensources. Commercially available distillers solubles such as Stimuflavsold by Hiram Walker and Sons is an example of such a nutrient. Some ofthe crude carbohydrate sources mentioned above, such as raw sugar beetjuice, apparently contain the trace elements and organic nitrogensources in sufficient quantity to make the addition of a nutrientunnecessary. It has been reported that good results have been obtainedwith raw sugar beet juice without the addition of a nutrient. Thefermentation is usually carried out at a temperature between about 70and 100F for aboutl to about 3 days after sterilizing the medium andinoculating it with bacteria of the genus Xanthomonas. Further detailsregarding the preparation of said biopolysaccharides can be found in US.Pat. Nos. 3,020,206, issued Feb. 6, 1962; 3,243,000 issued Mar. 29,1966; and 3,163,602 issued Dec. 29, I964. Polysaccharide B-l459 is anexample of a biopolysaccharide produced by the action of Xanthomonascampestris bacteria, and which is commercially available in variousgrades under the trademark Kelzan from the Kelco Company, Los Angeles,

Calif.

Included among the polyacrylamides which can be used in the practice ofthe invention are the various polyacrylamides and related polymers whichare waterdispersible and which can be used in an aqueous medium, withthe gelling agents described herein, to give an aqueous gel. Presentlypreferred polymers include the various substantially linear homopolymersand copolymers of acrylamide and methacrylamide. By substantially linearit is meant that the polymers are substantially free of crosslinkingbetween the polymer chains. Said polymers can have up to about 75,preferably up to about 45, percent of the carboxamide groups hydrolyzedto carboxyl groups. As used herein and in the claims, unless otherwisespecified, the term hydrolyzed includes modified polymers wherein thecarboxyl groups are in the acid form and also such polymers wherein thecarboxyl groups are in the salt form, provided said salts arewater-dispersible. Such salts include the ammonium salts, the alkalimetal salts, and others which are water-dispersible. Hydrolysis can becarried out in any suitable fashion, for example, by heating an aqueoussolution of the polymer with a suitable amount of sodium hydroxide.

Substantially linear polyacrylamides can be prepared by methods known inthe art. For example, the polymerization can be carried out in aqueousmedium, in the presence of a small but effective amount of awatersoluble oxygen-containing catalyst, e.g., a thiosulfate orbisulfate of potassium or sodium or an organic hydroperoxide, at atemperature between about 30 and 80C. The resulting polymer is recoveredfrom the aqueous medium, as by drum drying, and can be subsequentlyground to thedesired particle size. A presently 4 preferred particlesize is such that about weight percent will pass through a number 10mesh sieve, and not more than about 10 weight percent will be retainedon a 200 mesh sieve (U.S. Bureau of Standards Sieve Series).

Included among the copolymers which can be used to prepare gels for usein the practice of the invention are the water-dispersible copolymersresulting from the polymerization of a major proportion of acrylamide ormethacrylamide and a minor proportion of an ethylenically unsaturatedmonomer copolymerizable therewith. It is desirable that sufficientacrylamide or methacrylamide be present in the monomers mixture toimpart to the copolymer the above-described water-dispersibleproperties, for example, from about 60 to 99 percent acrylamide and fromabout 1 to 40 percent other ethyl enically unsaturated monomers,preferably from about 75 to percent acrylamide and from 5 to 25 percentother ethylenically unsaturated monomer. Such other monomers includeacrylic acid, methacrylic acid, vinylsulfonic acid, vinylbenzylsulfonicacid, vinylbenzenesulfonic acid, vinyl acetate, acrylonitrile, methylacryionitrile, vinyl alkyl ether, vinyl chloride, maleic anhydride,vinyl substituted cationic quaternary ammonium compounds, and the like.Various methods are known in the art for preparing said copolymers. Forexample, see US. Pat. Nos. 2,625,529; 2,740,522; 2,7.29,5 57; 2,831,841and 2,909,508. Said copolymers can also be used in the hydrolyzed form,as discussed above for the homopolymers.

Polyacrylic acids, including polymethacrylic acid, prepared by methodsknown in the art, can also be used to prepare gels for use in thepractice of the invention.

' Polyacrylates, e.g., as described in Kirk-Othmer, Encyclopedia ofChemical Technology, Vol. I, second edition, pages 305 et seq.,lnterscience Publishers, Inc., New York (1963), can also be used toprepare gels for use in the practice of the invention. Examples of saidpolyacrylates include polymers of methyl acrylate, ethyl acrylate,n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutylacrylate, tert-butyl acrylate, n-octyl acrylate, and the like.

Polymers of N-alkyl-substituted acrylamides wherein the nitrogen atomsin the carboxamide groups can have from 1 to 2 alkyl substituents whichcontain one to four carbon atoms can also be used to prepare gels foruse in the practice of the invention. Examples of said N-substitutedacrylamides include, among others, 'N-methyl acrylamide, N-propylacrylamide, N-butyl acrylamide, N,lN-dimethyl acrylamide,N-methyl-N-sec-butyl acrylamide, and the like, at various stages ofhydrolysis, as described above.

Crosslinked polyacrylamides and crosslinked polymethacrylamides, atvarious stages of hydrolysis as described above, can also be used toprepare gels for use in the practice of the invention. In general, saidcrosslinked polyacrylamides can be prepared by the methods describedabove, but including in the monomeric mixture a suitable amount of asuitable crosslinking agent. Examples of crosslinking agents includemethylenebisacrylamide, divinylbenzene, vinyl ether, divinyl ether, andthe like. Said crosslinking agents can be used in small amounts, e.g.,up to about 1 percent by weight of the monomeric mixture. Suchcrosslinking is to be distinguished from any crosslinking which occurswhen solutions of polymers are gelled as described herein.

All the polymers useful in preparing gels for use in the practice of theinvention are characterized by high molecular weight. The molecularweight is not critical so long as the polymer has the above-describedwaterdispersible properties. It is preferred that the polymer have amolecular weight of at least 100,000. The upper limit of molecularweight is unimportant so long as the polymer is water-dispersible, andthe aqueous gel prepared therefrom can be pumped. Thus, polymers havingmolecular weights as high as 20,000,000 or higher, and meeting saidconditions, can be used.

The first aqueous mixture will preferably have a polymer concentrationin the range of about 250 ppm to about 20,000 ppm and a multivalentmetal containing compound concentration in the range of about ppm toabout 2500 ppm. The water of the first mixture can be either fresh wateror brine. Generally speaking, fresh water is usually preferred whenavailable, as discussed further hereinafter The amount of theabove-described polymers used in preparing gels for use in the practiceof the invention can vary widely depending upon the particular polymerused, the purity of said polymer, and properties desired in said aqueousgels. In general, the amount of polymer used will be a water-thickeningamount, i.e., at least an amount which will significantly thicken thewater to which it is added. For example, amounts in the order of 25 to100 parts per million weight (0.0025 to 0.01 weight percent) have beenfound to significantly thicken water. Generally speaking, amounts of theabove-described polymers in the range of from 0.025 to 5, preferably0.025 to 2, weight percent, based on the weight of water, can be used inpreparing gels for use in the practice of the invention. However,amounts outside said ranges can be used. In general, with the properamounts of polyvalent metal and reducing agent, the amount of polymerused will determine the consistency of the gel obtained. Small amountsof polymer will usually produce liquid mobile gels which can be readilypumped. Large amounts of polymer will usually produce thicker, moreviscous, somewhat elastic gels. If desired, said thick gels can bethinned by dilution with water to any desired concentration of polymer.This can be done by mechanical means, e.g., stirring, pumping, or bymeans of a suitable turbulence inducing device to cause shearing, suchas a jet nozzle. Thus, there is really no fixed upper limit on theamount of polymer which can be used.

Multivalent metal compounds which can be used in the practice of theinvention are water-soluble compounds of polyvalent metals wherein themetal is present in a valence state which is capable of being reduced toa lower polyvalent state. Examples of such compounds include potassiumpermanganate, sodium permanganate, ammonium chromate, ammoniumdichromate, the alkali metal chromates, the alkali metal dichromates,and chromium trioxide. Sodium dichromate and potassium dichromate,because of low cost and ready availability are the presently preferredmetal-containing compounds for use in the practice of the invention. Thehexavalent chromium in said chromium compounds is reduced in situ totrivalent chromium by suitable reducing agents. In the permanganatecompounds, the manganese is reduced from +7 valence to +4 valence, as inMnO The amount of said metal-containing compounds used will be asensible amount, i.e., a small but finite amount which is more thanincidental impurities, but

6 which is effective or sufficient to cause subsequent gelation when themetal in the polyvalent metal compound is reduced to a lower polyvalentvalence state. The lower limit of the concentration of the startingmetal-containing compound will depend upon several factors including theparticular type of polymer used, the concentration of the polymer in thewater to be gelled, the water which is used, and the type of gel productdesired. For similar reasons, the upper limit on the concentration ofthe starting metal-containing compound also cannot always be preciselydefined. However, it should be noted that excessive amounts of thestarting metal compound, for example +6 chromium, which can lead toexcessive amounts of +3 chromium when there is sufficient reducing agentpresent to reduce the excess +6 chromium, can adversely affect thestability of the gels produced. As a general guide, the amount of thestarting polyvalent metal-containing compound used in preparing aqueousgels for use in the practice of the invention will be in the range offrom 0.05 to 40, preferably 0.5 to 40, more preferably 2 to 30, weightpercent of the amount of the polymer used. However, in some situationsit may be desirable to use amounts of the starting polyvalentmetal-containing compound which are outside the above ranges. Such useis within the scope of the invention. Those skilled in the art candetermine the amount of starting polyvalent metal-containing compound tobe used by simple ex periments carried out in the light of thisdisclosure. For example, when brines, such as are commonly available inproducing oil fields, are used in the water in preparing gels for use inthe practice of the invention, less of the starting polyvalentmetal-containing compound is required than when distilled water is used.Suitable gels can be prepared using brines having a wide range ofdissolved solids content, depending upon the particular polymer andbrine used. Gelation rates are frequently faster when using said brines.Such oil field brines commonly contain varying amounts of sodiumchloride, calcium chloride, magnesium chloride, etc. Sodium chloride isusually present in the greatest concentration.

Generally speaking, water having a low (or essentially none) totaldissolved solids content is the preferred medium for preparing the gelsdescribed herein. Generally speaking, when brines are used suitable gelscan be obtained when the total dissolved solids content is not greaterthan about 60,000, and the amount of polyvalent metal ions such ascalcium, magnesium, etc., is not greater than about 6,000.

The above-described first mixture is thereafter mixed with a complexingagent to form a second mixture. Examples of complexing agents which canbe used in the practice of this invention include, among others,citrates, tartrates, acetates, phosphates, hydrophosphites, orarsenates, for example, citric acid, ferric hydrophosphite, aluminumacetate, sodium potassium tartrate, calcium arsenate, and sodiumphosphate. Organic complexing agents can also be used in the practice ofthe invention. An example of such an organic complexing agent is thegreen food coloring material identified as the disodium salt of 4-[4-(N-ethyl-p-sulfobenzylamino )-phenyl 4-hydroxy-2 -sulfoniumphenyl)-methylene}-[1-(N-ethyl-N-p-sulfobenzyl)A cyclohexadienimine The secondmixture can have a complexing agent concentration in the range of about5 ppm to about 5000 ppm. The amount of said complexing agents used inthe practice of the invention will be an amount which is sufficient todelay or increase the gelling time of the polymer solution in which saidagent is used. Thus, the amount of complexing agent used will, ingeneral, depend upon such related factors as how much delay in gellingor increase in gelling time is desired, the particular complexing agentused, the type and depth of the formation being treated, the amount ofstarting multivalent metal-containing compound used, etc. Thus, thereare no real fixed limits on the amount of said complexing agents used.However, as a guide to those skilled in the art, the amount of saidcomplexing agent used will usually be within the range of from 10 to 300weight percent of the weight of said starting multivalentmetal-containing compound which is used. In most instances, it will bepreferred to use an amount of complexing agent which is within the rangeof from 50 to 250 weight percent of the weight of said startingmultivalent metal-containing compound. However, it is within the scopeof the invention to use amounts of said complexing agents which areoutside said ranges. It will usually be desirable to use sufficient ofsaid complexing agent to delay or extend the gelling time sufficient topermit the solution to be pumped to the bottom of the borehole and startinto the formation before significant gelation occurs, e.g., anextension of time in the order of up to about one hour, or longer,depending upon borehole depth and the formation being treated.

The second mixture is thereafter mixed with a reducing agent to form athird mixture. Said reducing agent reduces the valence of at least aportion of the metal ions to a lesser valence, and at least a portion ofthe newly reduced metal ions are apparently complexed by the complexingagent, as discussed further hereinafter. Examples of reducing agentswhich can be used in the practice of the invention includesulfur-containing compounds such as sodium sulfite, sodium hydrosulfite,potassium hydrosulfite, sodium metabisulfite, potassium sulfite, sodiumbisulfite, potassium bisulfite, potassium metabisulfite, sodium sulfide,potassium sulfide, sodium thiosulfate, potassium thiosulfate, ferroussulfate, thioacetamide, and others; and nonsulfur-containing compoundssuch as hydroquinone, ferrous chloride, p-hydrazinobenzoic acid,hydrazine phosphite, hydrazine dichloride, and others.

The third mixture can have a reducing agent concentration in the rangeof about ppm to about 2500 ppm. The amount of reducing agent to be usedin the practice of the invention will be a sensible amount, i.e., asmall but finite amount which is more than incidental impurities, butwhich is effective or sufficient to reduce at least a portion of thehigher valence metal in the starting polyvalent metal-containingcompound to a lower polyvalent valence state. Thus, the amount ofreducing agent to be used depends, to some extent at least, upon theamount of the starting polyvalent metalcontaining compound which isused. In many instances, it will be preferred to use an excess ofreducing agent to compensate for dissolved oxygen in the water, exposureto air during preparation of the gels, and possible contact with otheroxidizing substances such as might be encountered in field operations.As a general guide, the amount of reducing agent used will generally bewithin the range of from 0.1 to at least 150, preferably at least about200, weight percent of the stoichiometric amount required to reduce themetal in the starting polyvalent metal-containing compound to said lowerpolyvalent valence state, e.g., +6 Cr to +3 Cr. However, in someinstances, it may be desirable to use amounts of reducing agent outsidesaid ranges. The use of such amounts is within the scope of theinvention. Those skilled in the art can determine the amount of reducingagent to be used by simple experiments carried out in the light of thisdisclosure.

It has been discovered that this process is particularly useful incrosslinking polymers in brine solutions which have the properties ofshort gel times. It has also been discovered that if a polymer solutiongels rapidly, the resultant gel will sometimes have a lower stabilityand quality than the gelled polymer solutions of this process.

The reaction mechanism(s) involved in the action of said complexingagents in delaying or extending the gelation time of polymer soultionsin accordance with the invention is not completely understood at thistime. While it is not intended to limit the invention by any theories asto the nature of said reaction mechanism(s), it is presently believedthat the above-described complexing agents, e.g., citric acid, in somemanner complex with, chelate, or otherwise react with, at least aportion of the newly reduced polyvalent metal ions. It is believed saidcomplexing agents are thus in competition with the polymer for saidnewly reduced polyvalent metal ions, and temporarily reduce the numberof said ions available for reaction with, e.g., crosslinking, thepolymer. Apparently some sort of equilibrium is established, said newlyreduced metal ions are slowly released from their reaction product withsaid complexing agent and made available for reaction with said polymer,and the gelation time of the polymer solution is thus delayed orextended. Thus, herein and in the claims, unless otherwise specified,the terms complex, complexed, complexing, when employed in connectionwith the above-described complexing agents and the action thereof, areemployed generically to include any reaction mechanism(s) by which saidagents react with said newly reduced metal ions to provide said delay orextension in gelling time.

In the method of this invention it is essential that the complexingagent be mixed with the polymer solution which contains the multivalentmetal-containing compound prior to adding the reducing agent in order tomaintain desirable gel time control. If the reducing agent is added tothe first mixture with or before the complexing agent, there will beless control over the gel time and a gel can be formed which has aviscosity greater than desirable. Where the viscosity of the gelledpolymer solution being passed into the formation is high, it is moredifficult to readily and easily distribute the gelled solution throughthe flow channels. In some cases where gelation has progressed rapidly,the viscosity of the gelled solution can be so great that the gel cannotbe passed into the formation.

In one embodiment of the invention the third mixture is passeddownwardly through a well bore and into and through the subterraneanformation to a desired location. Pumping of materials into the well boreis there after terminated and the polymer is maintained at the desiredlocation until the materials gel. Thereafter the well is placed back inoperation. Other embodiments of the invention include using said thirdmixture to decrease the mobility of a drive fluid, or decrease thepermeability of a formation, in secondary recovery processes. Furtherdetails of the processes comprising said other embodiments can be foundin US. Pat. No. 3,727,687. See particularly column 7, line 47 to col- 3,9 26,25 8 9 10 umn 8, line 68. ganic complexing agent used was the sameas used in The following are examples of preferred methods of ExampleII. this invention and the gelled solutions of this invention comparedto heretofore utilized methods.

EXAMPLE 1v EXAMPLE I Comparative Runs V1scos1ty, cp at 170 sec Firstmixture comprises fresh water, 5000 ppm CMC m 911, and 1250 ppm of Na CrO 211,0. 2 3 4 Material added to said first mixture to form a second 8g2? 2 :2 mixture was 2500 ppm citric acid. 10 10 150 70 85 55 Materialadded to said second mixture to form a third 265 85 130 105 mixture was1250 ppm sodium bisulfite. 38 8 :58 88 A control solution identical tothe above solution was 30 760 255 600 715 formed except the citric acidwas omitted. Said control 35 S8 33g 388 388 solution began to thicken inabout 15 minutes. The 15 12 520 above solution of this inventioncontaining the citric 50 615 acid began to thicken in about 25 minutes.This differas I 3% ence in gelling time period is exceedingly importantin 65 900 situations where the material must be pumped into a formationwhich is located several thousand feet below the ground surface.Compositions tested: All four contained 5000 ppm CMC 9H, 1250 ppm Na CrO .2H O, and 1250 ppm EXAMPLE n NaHSO in fresh water. Said fourcompositions differed A 3000 ppm solution of CMC 9I-I was mixed insimuonly in whether or not they contained citric acid, and in latedArbuckle formation brine water containing order of addition as set forthbelow.

Run 1 (a) 1250 ppm Na Cr O,-2H O Run 3 (a) 1250 ppm Na Cr O-,'2H O (b)I250 ppm NaHSO (b) I250 ppm NaHSO (c) No citric acid (c) 2500 ppm citricacid (added within seconds of NaHSO Run 2 (a) 1250 ppm N21 Cr O '2H ORun 4 (a) I250 ppm Na Cr O,'2H O (b) I250 ppm NaI-ISO (b) 1250 ppmNaHSO;

(c) 2500 ppm citric acid (added (c) 2500 ppm citric acid (added beforethe NaHSO at same time as the NaI-ISO;,)

47,000 ppm total dissolved solids. A portion of the so- The above fourdifferent methods of mixing the inlution was gelled with 75 ppm ofsodium dichromate gredients used in preparing the gelled solutions ofthe and 75 ppm of sodium hydrosulfite. The solution gelled inventionhave been tested. The method of the above within 3 minutes. Run 2 is theprocedure for mixing said ingredients in An identical second portion ofsaid CMC solution 40 accordance with this invention. In the control testwas gelled with the same amounts of said gelling agents shown in Run 1of the above table, the polymer, the reexcept that 2 drops of green foodcoloring per 100 ml ducing agent, and the oxidizing agent were mixedtoof polymer solution was added before the reducing gether (no citricacid) and the viscosity measured in agent was added. The green foodcoloring (organic centipoise at 170 sec employing a Fann Model 35viscomplexing agent) used was a solution reported to concosimeter. InRun 1, a gel having 900+ centipoise after tain water, propylene glycol,and 2.5 percent of the di- 40 minutes was obtained. The reason the testwas sodium salt of 4-{ [4-(N-ethyl-p-sulfobenzylamino)- stopped at 900centipoise is that this is the limit of thephenyl]-(4-hydroxy-2-sulfoniumphenyl)-methylene -l viscosimeter whichwas utilized for the measurements. [l-(N-ethyl-N-p-sulfobenzyl)- A-cyclohexadieni- Run 3 was made using the same basic solution as inmine]. This solution gelled in about 20 minutes. With Run 1 except that2500 ppm citric acid was added the 20-minute delay in gelation obtainedwith said within 30 seconds after the NaHSO was added. The green foodcoloring, we were able to inject a 3000 ppm viscosity of this solutionattained 900 centipoise in 40 polymer solution into an Ottawa sand pack.After an minutes. overnight set period we were unable to inject anybrine Run 4 was made using the same basic solution as Run into the sandpack at 100 psi. Prior to this invention a 3 except that the NaI-ISO andcitric acid were added at 1000 ppm gelled solution of polymer was themaximum the same time. In this run a 900 centipoise viscosity wasinjectable into a comparable sand pack. attained in 40 minutes.

These three runs, namely Runs 1, 3, and 4, demon- EXAMPLE m stratedifferent ways of mixing the acid (or no acid) A 5000 ppm solution ofCMC 9H was prepared in tap into the solution. Said three runsdemonstrate that Water and gelation thereof was caused by adding therewas no appreciable difference in the gelling time thereto 1250 ppmsodium dichromate and 1250 ppm of of the polymer solution and, in fact,no difference in sodium bisulfite, Gel time was about 15 minutes.Angelling time to 900 CP viscosity when citric acid was other solutionwas prepared using the same amounts of not used, as in Run 1. chemicals,but 3 cc/liter of said solution of the above Run 2 demonstrates themethod of the present invengreen food coloring was added before addingthe sotion in that the complexing agent, e.g., citric acid, was diumbisulfite. The solution began to thicken in one added before thereducing agent, e.g., NaI-ISO In this hour and gelled solid overnight.The food coloring orparticular instance, the 900 centipoise viscositywas attained in 65 minutes, i.e., it required 25 more minutes for thesolution to gel to the same viscosity than was required in Runs 1, 3,and 4. It should also be noted that in Run 2 after 30 minutes theviscosity of the solution was only 255 CP, whereas in Runs 1, 3, and 4the viscosity was at least 600 CP. Run 2 clearly demonstrates theoperability and practicability of this invention in delaying orextending the gelling time of the CMC solution. The delay or extensionin gelling time can be varied by varying the amount of complexing agent,e.g., citric acid, which is mixed into the solution prior to theaddition of the reducing agent. Different amounts of reducing agent andoxidizing agent can be used, as well as varying the amount of polymer,e.g., CMC, used in particular combinations, to produce variations in thegelling time of polymer solutions. The present tests, however,definitely indicate that when a delay or extension in gelling time isdesired, it can be achieved when a complexing agent, e.g., citric acid,is placed in the solution prior to the addition of the reducing agent.

Other modifications and alterations of this invention will becomeapparent to those skilled in the art from the foregoing discussion andexamples and it should be understood that this invention is not to beunduly limited to said examples.

We claim:

l. A method for altering the permeability of a subterranean formationwith a gelled solution of a polymer having an increased gel time, whichmethod comprises:

mixing (a) one of a polyacrylamide, a polysaccharide, a cellulosicpolymer, or mixtures thereof, (b) t a compound of a multivalent metalcapable of furnishing multivalent metal ions, and water, to form a firstmixture;

thereafter mixing a complexing agent with said first mixture to form asecond mixture;

thereafter forming a third mixture by mixing with said second mixture areducing agent capable of reducing at least a portion of saidmultivalent metal ions to a reduced valence state and cause gelation ofsaid solution; said complexing agent being capable of complexing with atleast a portion of said reduced multivalent metal ions to provide saidincreased gel time; and

passing said third mixture having an increased gel time into saidformation.

2. A method according to claim 1 wherein said first mixture is anaqueous solution having a polymer concentration in the range of about250 ppm to about 20,000 ppm, and said multivalent metal compound issodium dichromate present in a concentration in the range of about ppmto about 2500 ppm.

3. A method according to claim 1 wherein the water of said first mixtureis one of fresh water on brine.

4. A method according to claim 1 wherein said complexing agent is one ofcitrate, acetate, or arsenate.

5. A method according to claim 1 wherein said complexing agent is citricacid.

6. A method according to claim 1 wherein said second mixture has acomplexing agent concentration in the range of about 5 ppm to about 5000ppm.

7. A method according to claim 1 wherein said reducing agent is sodiumhydrosulfite.

8. A method according to claim 1 wherein said third mixture has areducing agent concentration in the range of about 5 ppm to about 2500ppm.

9. A method according to claim 1 wherein:

said polymer is a carboxymethyl cellulose and the concentration thereofin said first mixture is within the range of from 250 to 20,000 ppm;said multivalent metal compound is sodium dichromate and theconcentration thereof in said first mixture is within the range of from10 to 25 00 pp said complexing agent is citric acid and theconcentrations thereof in said second mixture is within the range offrom 5 to 5,000 ppm; and said reducing agent is sodium hydrosulfite orsodium bisulfite and the concentration thereof in said third mixture iswithin the range of from 5 to 2500 ppm. 10. In a method wherein anaqueous gel is introduced into a borehole in the earth and into asubterranean formation penetrated by said borehole, the improvementwherein said aqueous gel has an extended gelation time and compriseswater to which there has been added:

a water-thickening amount of a water-dispersible polymer selected fromthe group consisting of (a) water-soluble cellulose ethers, (b)polyacrylamides and polymethacrylamides wherein up to about percent ofthe carboxamide groups can be hydrolyzed to carboxyl groups; crosslinkedpolyacrylamides and crosslinked polymethacrylamides wherein up to about75 percent of the carboxamide groups can be hydrolyzed to carboxylgroups; polyacrylic acid and polymethacrylic acid; polyacrylates;polymers of N-substituted acrylamides wherein the nitrogen atoms in thecarboxamide groups can have from 1 to 2 alkyl substituents which containfrom 1 to 4 carbon atoms; copolymers of acrylamide with anotherethylenically unsaturated monomer copolymerizable therewith, sufficientacrylamide being present in the monomer mixture to impart saidwater-dispersible properties to the resulting copolymer when it is mixedwith water, and wherein up to about 75 percent of the carboxamide groupscan be hydrolyzed to carboxyl groups; and mixtures thereof; and (c) abiopolysaccharide produced by the action of bacteria of the genusXanthomonas oncarbohydrates; an amount of a water-soluble compound of apolyvalent metal wherein the metal present is capable of being reducedto a lower polyvalent valence state and which amount is sufficient tocause gelation of said water containing said polymer when the valence ofat least a portion of said metal is reduced to said lower valence state;an amount of a complexing agent capable of complexing with ions of saidmetal in said reduced valence state; and sufficient to complex with atleast a portion of said ions and cause said extended gelation time; andan amount of a water-soluble reducing agent which is effective to reduceat least a portion of said metal to said lower valence state, with saidreducing agent being added to said water after said complexing agent hasbeen added thereto. lll. A methodaccording to claim 10 wherein there hasbeen added to said water:

from 0.025 to 5 weight percent of said cellulose ether, based'upon theweight of said water; from 0.05 to 40 weight percent of said compound ofapolyvalent metal, based upon the weight of said cellulose ether;

from to 300 weight percent of said complexing agent, based upon theweight of said polyvalent metal compound; and

from 0.1 to at least about 200 percent of the stoichiometric amount ofsaid reducing agent required to reduce said polyvalent metal to saidlower polyvalent valence state.

12. A method according to claim 11 wherein:

said cellulose ether is a carboxymethyl cellulose ether;

said polyvalent metal compound is a compound of chromium selected fromthe group consisting of ammonium chromate, ammonium dichromate, thealkali metal chromates and dichromates, chromium trioxide, and mixturestherof; and

said reducing agent is selected from the group consisting of sodiumsulfide, sodium hydrosulfite, sodium metabisulfite, sodium bisulfite,sodium thiosulfate, potassium sulfide, potassium hydrosulfite, potassiummetabisulfite, potassium bisulfite, potas sium thiosulfate, and mixturesthereof.

13. A method according to claim 12 wherein:

said cellulose ether is sodium carboxymethyl cellulose;

said polyvalent metal compound is sodium dichromate; and

said complexing agent is citric acid.

14. A method according to claim 13 wherein:

from 0.025 to 2 weight percent of said cellulose ether has been added tosaid water;

from 2 to 30 weight percent of said sodium dichromate has been added tosaid water; and

from 50 to 250 weight percent of said citric acid has been added to saidwater.

1. A METHOD FOR ALTERING THE PERMEABILITY OF A SUBTERRANEAN FORMATIONWITH A GELLED SOLUTION OF A POLYMER HAVING AN INCREASED GEL TIME, WHICHMETHOD COMPRISES: MIXING (A) ONE OF A POLYACRYLAMIDE, A POLYSACCHARIDE,A CELLULOSIC POLYMER, OR MIXTURES THEREOF, (B) A COMPOUND OF AMULTIVALENT METAL CAPABLE OF FURNISHING MULTIVALENT THEREAFTER MIXNG ACOMPLEXING AGENT WITH SAID FIRST MIXTURE METAL IONS, AND (C) WATER, TOFORM A FIRST MIXTURE; TO FORM A SECOND MIXTURE; THEREAFTER FORMING ATHIRD MIXTURE BY MIXING WITH SAID SECOND MIXTURE A REDUCING AGENTCAPABLE OF REDUCING AT LEAST A PORTION OF SAID MULTIVALENT METAL IONS TOA REDUCED VALENCE STATE AND CAUSE GELATION OF SAID SOLUTION; SAIDCOMPLEXING AGENT BEING CAPABLE OF COMPLEXING WITH AT LEAST PORTION OFSAID REDUCED MULTIVALENT METAL IONS TO PROVIDE SAID INCREASE GEL TIME;AND PASSING SAID THIRD MIXTURE HAVING AN INCREASED GEL TIME INTO SAIDFORMATION.
 2. A method according to claim 1 wherein said first mixtureis an aqueous solution having a polymer concentration in the range ofabout 250 ppm to about 20,000 ppm, and said multivalent metal compoundis sodium dichromate present in a concentration in the range of about 10ppm to about 2500 ppm.
 3. A method according to claim 1 wherein thewater of said first mixture is one of fresh water on brine.
 4. A methodaccording to claim 1 wherein said complexing agent is one of citrate,acetate, or arsenate.
 5. A method according to claim 1 wherein saidcomplexing agent is citric acid.
 6. A method according to claim 1wherein said second mixtuRe has a complexing agent concentration in therange of about 5 ppm to about 5000 ppm.
 7. A method according to claim 1wherein said reducing agent is sodium hydrosulfite.
 8. A methodaccording to claim 1 wherein said third mixture has a reducing agentconcentration in the range of about 5 ppm to about 2500 ppm.
 9. A methodaccording to claim 1 wherein: said polymer is a carboxymethyl celluloseand the concentration thereof in said first mixture is within the rangeof from 250 to 20,000 ppm; said multivalent metal compound is sodiumdichromate and the concentration thereof in said first mixture is withinthe range of from 10 to 2500 ppm; said complexing agent is citric acidand the concentrations thereof in said second mixture is within therange of from 5 to 5,000 ppm; and said reducing agent is sodiumhydrosulfite or sodium bisulfite and the concentration thereof in saidthird mixture is within the range of from 5 to 2500 ppm.
 10. In a methodwherein an aqueous gel is introduced into a borehole in the earth andinto a subterranean formation penetrated by said borehole, theimprovement wherein said aqueous gel has an extended gelation time andcomprises water to which there has been added: a water-thickening amountof a water-dispersible polymer selected from the group consisting of (a)water-soluble cellulose ethers, (b) polyacrylamides andpolymethacrylamides wherein up to about 75 percent of the carboxamidegroups can be hydrolyzed to carboxyl groups; crosslinked polyacrylamidesand crosslinked polymethacrylamides wherein up to about 75 percent ofthe carboxamide groups can be hydrolyzed to carboxyl groups; polyacrylicacid and polymethacrylic acid; polyacrylates; polymers of N-substitutedacrylamides wherein the nitrogen atoms in the carboxamide groups canhave from 1 to 2 alkyl substituents which contain from 1 to 4 carbonatoms; copolymers of acrylamide with another ethylenically unsaturatedmonomer copolymerizable therewith, sufficient acrylamide being presentin the monomer mixture to impart said water-dispersible properties tothe resulting copolymer when it is mixed with water, and wherein up toabout 75 percent of the carboxamide groups can be hydrolyzed to carboxylgroups; and mixtures thereof; and (c) a biopolysaccharide produced bythe action of bacteria of the genus Xanthomonas on carbohydrates; anamount of a water-soluble compound of a polyvalent metal wherein themetal present is capable of being reduced to a lower polyvalent valencestate and which amount is sufficient to cause gelation of said watercontaining said polymer when the valence of at least a portion of saidmetal is reduced to said lower valence state; an amount of a complexingagent capable of complexing with ions of said metal in said reducedvalence state; and sufficient to complex with at least a portion of saidions and cause said extended gelation time; and an amount of awater-soluble reducing agent which is effective to reduce at least aportion of said metal to said lower valence state, with said reducingagent being added to said water after said complexing agent has beenadded thereto.
 11. A method according to claim 10 wherein there has beenadded to said water: from 0.025 to 5 weight percent of said celluloseether, based upon the weight of said water; from 0.05 to 40 weightpercent of said compound of a polyvalent metal, based upon the weight ofsaid cellulose ether; from 10 to 300 weight percent of said complexingagent, based upon the weight of said polyvalent metal compound; and from0.1 to at least about 200 percent of the stoichiometric amount of saidreducing agent required to reduce said polyvalent metal to said lowerpolyvalent valence state.
 12. A method according to claim 11 wherein:said cellulose ether is a carboxymethyl cEllulose ether; said polyvalentmetal compound is a compound of chromium selected from the groupconsisting of ammonium chromate, ammonium dichromate, the alkali metalchromates and dichromates, chromium trioxide, and mixtures therof; andsaid reducing agent is selected from the group consisting of sodiumsulfide, sodium hydrosulfite, sodium metabisulfite, sodium bisulfite,sodium thiosulfate, potassium sulfide, potassium hydrosulfite, potassiummetabisulfite, potassium bisulfite, potassium thiosulfate, and mixturesthereof.
 13. A method according to claim 12 wherein: said celluloseether is sodium carboxymethyl cellulose; said polyvalent metal compoundis sodium dichromate; and said complexing agent is citric acid.
 14. Amethod according to claim 13 wherein: from 0.025 to 2 weight percent ofsaid cellulose ether has been added to said water; from 2 to 30 weightpercent of said sodium dichromate has been added to said water; and from50 to 250 weight percent of said citric acid has been added to saidwater.